1. Field of the Invention
The present invention relates to an ignition switch key cylinder of a vehicle, and more particularly, to a key cylinder incorporating a coil serving as an antenna of a receiver.
2. Description of the Related Art
Some vehicles are equipped with anti-theft engine immobilizer systems. FIG. 7 shows an engine immobilizer system. A transponder 52 is incorporated in an ignition key 51 and transmits a signal indicative of an ID codes. A receiver is coupled to an engine start control computer incorporated in the vehicle. A coil antenna 55 is provided around a cap member 54, which is secured to a key cylinder 53. The receiver receives the ID code from the transponder 52 via the coil antenna 55. The receiver sends the received code to the computer. When the received ID code and an identification code stored therein do not match, the computer stops providing fuel to the engine. Therefore, the engine runs for a limited period of time when it is started by the ignition key 51. Therefore, a key having no transponder, if capable of starting the engine, cannot keep the engine running. Car theft is thus prevented.
The coil antenna 55 is constituted by a resin bobbin 56 and a coil 57. The coil 57 is wound around the bobbin 56. The bobbin 56 and the coil 57 are molded with resin. As shown in FIG. 8, a connector 58 and a tang 59 are formed integrally with the coil antenna 55. The coil antenna 55 is electrically coupled to the receiver via the connector 58.
As shown in FIG. 8, the key cylinder 53 is inserted in a substantially cylindrical lock body 60. The lock body 60 is incorporated in a steering handle lock (SHL). The key cylinder 53 is fixed in the lock body 60 by engaging a projection 61, which is provided on the cylinder 53 and biased outwardly by a spring, to a hole 62 formed in the lock body 60. The coil antenna 55 is fitted to the case 54 of the key cylinder 53. The antenna 55 is then secured to the lock body 60 by fastening a screw 63 to the body 60 through the tang 59. The coil antenna 55 and the key cylinder 53 are indirectly secured to each other by means of the screw 63 and the lock body 60.
The transponder 52 is a transmitter requiring no battery. When excited by the magnetic field generated by the coil 57 at a predetermined frequency, the transponder 52 is electrically charged. The transponder 52 then transmits an encrypted code using the charged electricity at a predetermined frequency. The frequency at which the transponder 52 is excited and the transmission frequency of the transponder 52 are previously determined. The characteristic values of the coil antenna 55, such as L (inductance) and Q values of the coil 57, play an important role when setting the resonance frequency of the receiver. The characteristic value of the antenna 55 varies depending upon the relative position of the coil 57 to a proximate metal object (the zinc die-cast case 54 in FIG. 5). In this case the characteristic values of the antenna 55 vary depending on the distance between the coil 57 and the case 54. A measuring and testing of the antenna characteristic value is performed with the coil antenna 55 temporarily mounted on the lock body 60. Only the coil antennas 55 having a characteristic value within a desirable range are shipped after the measuring and testing.
However, when assembling the coil antenna 55 to the lock body 60 on an assembly line, the position of the antenna 55 relative to the key cylinder 53, i.e., the distance between the antenna 55 and the cylinder 53, cannot be set exactly the same as the position in the measuring and testing. The antenna position also differs from one vehicle to another. Furthermore the relative position of the coil antenna 55 to the key cylinder 53 changes as the screw 63 is loosened by vibrations and heat cycles. These changes of the coil antenna position relative to the key cylinder 53 may result in the characteristic values of the coil antenna 55 being out of the desirable range.
Reducing the tolerance of parts dimension is a way to reduce the changes and variations of the distance between the coil antenna 55 and the key cylinder 53. This, however, results in a reduction of play between the parts and therefore makes the assembly difficult. Furthermore, a reduced parts tolerance, or enhanced accuracy of parts, increases the manufacturing cost.